Position and time information via free space optical communications

Abstract

The development of large Low Earth Orbit (LEO) satellite constellations is driving the need for autonomous satellite orbit determination. The present state-of-the art is the use of global positioning system (GPS) receivers on individual satellites that provide the position and time information necessary to determine the satellite orbit. There is a growing realization of potential threats to GPS and Global Navigation Satellite System (GNSS) constellations in general. Loss of GNSS service would have significant implications to these large satellite constellations. At the time of writing, the Galileo GNSS system has been down for a week, and there is very little public information as to why and how long before services will be restored. Free Space Optical Communications (FSOC) between satellites and satellite-to-ground links has the potential to provide an independent source for position and time data necessary for orbit determination. FSOC systems can provide highly accurate ranging and time synchronization between both ends of an FSOC link. In the case of pulse-position-modulation (PPM), commonly used in FSOC systems, precise range and time synchronization is required to make the link work properly. The combination of inter-satellite ranging and ranging to fixed ground stations with access to precise timing can provide significant GPS and GNSS independent autonomous orbit determination of spacecraft and spacecraft constellations.